1. Field of the Invention
The invention relates to defect detection in articles using computer modelled dissipation correction differential time delay Far infra-red scanning. Especially the invention relates to such defect detection in articles such as fibre board panels, oriented strand board panels, medium density fibre board panels, metal panels, metal pipes, coated metal pipes and similar articles.
2. Acknowledgement of the Prior Art
Non-destructive testing inspection using Far IR scanning is well known in the detection of hot spots for example detecting where insulation is absent, where friction components are malfunctioning, or where cooling/exhaust systems are failing. However, flaws which do not cause local hot spots are more difficult to detect. Some of these flaws are very hard to detect.
Various attempts have been made to overcome the difficulties which arise in this type of scanning for flaws. Examples of methods which have been used are set out in U.S. Pat. No. 5,357,112 issued Oct. 18, 1994 to Steele et al., U.S. Pat. No. 5,444,241 issued Aug. 22, 1995 to Del Grande et al., and U.S. Pat. No. 5,631,465 issued May 20, 1997 to Shepard.
The horizontal density variation of Oriented Strand Board (OSB) affects most of the physical and mechanical properties of the panel. Between-panel density variation can well be measured and controlled. Within-panel variation, however, has been difficult to measure. A better estimation of this horizontal density variation obviously could provide information for controlling the mat forming process to reduce density variation. A more uniform density distribution would allow for a reduction in panel thickness or density, which would eventually improve wood fibre utilization.
Destructive measurements of OSB panel density are usually slow and expensive in terms of labor cost. There is a need for methods of nondestructive measurements of OSB density both at laboratory and industrial scales.
It has been observed qualitatively that variation in density could be detected using a Far infra-red imaging system. IR thermography technology was used to estimate OSB panel density.
The fact that radiation is a function of object surface temperature makes it possible for an IR camera to calculate and display this temperature. If an OSB panel contains an anomaly in its density, and the panel starts at an initial uniform temperature, then as it is quickly heated and cooled, the anomaly will produce an anomaly in the distribution of surface temperature. This is because, in the course of temperature change, those areas of the panel which have lower density will lose or gain heat more rapidly and high density areas lose or gain heat more slowly. This is the basic theoretical principle on which infrared OSB density measurement is based. An aim of this invention was to determine the accuracy, spatial resolution and speed of IR measurement of OSB panel density.
It has also been surprisingly discovered that in a large central area of an article it is not necessary to resort to various precautions to overcome difficulties. It is only necessary to utilize precautions in a marginal area where cooling of an unflawed article does not occur in such a set pattern as in a central area.
The present invention provides a process for the detection of flaws in an article, especially OSB, using Far infra-red scanning of its surface comprising changing the temperature of the surface of an article over a plurality of temperatures and making an infra-red scan at each of said temperatures during changing the temperature, the infra-red scans being separated one from another by equal time increments; characterized in the steps of allocating parts of said surface as central and marginal parts forming images from said infra-red scans, digitizing said infra-red scans, digitizing the images to provide a sequence of digitized scanned images; for said central part of the surface, comparing data directly from said digitized scanned images and noting variations and/or anisotropies from a general cooling pattern for the article and deducing the presence of flaws at locations in the article corresponding to the location of the variations and/or anisotropies in the comparison of the digitized scanned images; and for the marginal part of the surface, performing thermodynamic modelling on one of the digitized scanned images to provide an estimate of the temperature distribution for a hypothetic unflawed article after passage of one of said time increments, and comparing data from an adjacent digitized scanned image with said estimate and noting variations and/or anisotropies of the structure of the marginal part of the article.
The present invention also provides a process for detection of flaws in an article, especially in OSB. This process comprises changing the temperature of the surface of an article over a plurality of temperatures; making an infra-red scan at each of said temperatures during changing of temperature; said infra-red scans providing at least a first and a second scanned image and being separated one from the other by a time increment; digitizing the at least first and second scanned images to provide a sequence of at least a first and a second digitized scanned image; performing thermodynamic modelling on the first digitized scanned image to provide an estimate of the temperature distribution for a hypothetic unflawed article after passage of said time increment; comparing data from said second digitized image with said estimate, noting variations and/or anisotropies of the structure of the article. Thereafter, quality decisions about the fitness of the article can be made.
While first and second scans at first and second temperatures may be sufficient to provide data for flaw detection, a group of scans may be made at a series of three or more temperatures for greater accuracy. Said thermodynamic estimate may be made at any one of this series of temperatures and may be compared with data from scanned images obtained at higher or lower temperatures.
The relative proportion of the central and marginal parts may be chosen in accordance with the shape and size of the article, the material from which it is made and the degree of accuracy required. For example, if the article is a circular metal plate of say 10 feet in diameter, the central portion may be a 9 foot circle within an annular marginal portion. If the plate is formed of a less thermally conductive material, the marginal portion may be smaller. If, however, the plate is square, the central portion may possibly still be circular, since the corners of the square cause irregularities. Many of the decisions will be within the skill of the operator once the general principle is appreciated may be made by a man skilled in the art. In very general terms the central portion may be of regular shape and may be from 10-90% of the surface area of the article.
More particularly the central part may be from 20-80% or especially 75% of the surface area of the article.
The process of specifically inducing, or introducing a heating or cooling transient, with the specific intention of creating a temporary temperature differential in what would have otherwise been a steady state situation is particularly important. The creation of, high speed monitoring of, image acquisition of, image processing of, enhancement of, and thermodynamic modelling of, these temporary temperature differences constitutes the essence of this invention.
Conveniently the thermodynamic modelling and the comparison of data are performed by a suitably programmed computer.
In the following specific detailed discussion, it is always assumed that a surface of the article to be tested is heated above ambient temperature and allowed to cool. In fact, it is within the scope of the invention to cool the article below ambient temperature and allow it to heat up to obtain two incremental temperature differences.
While the following detailed discussion is limited to the scanning and comparison of only two images at different temperatures, it is clear that a much larger number of images may be scanned and compared.
For example, the process may comprise the following steps:
1. Central and marginal parts are designated if desired.
2. The component to be inspected is heated so that its temperature rises significantly above ambient temperature. This heating is preferably uniform, and preferably of at least 50 degrees Celsius in magnitude.
3. An IR image of the surface of the heated component to be analysed is obtained with sufficient resolution (in temperature, spatial, and temporal domains) to allow for detection of defects. The spatial resolution required will depend on the defects in question (for example variation in oriented strand board (OSB) panels might require resolution of xc2xcxe2x80x3 square, variations in pipe wall thickness might require resolution of 0.5 mm square). The temperature resolution required from the Far IR image will typically be from 0.1 to 0.2 degrees Celsius. Typically the scanner will be a forward looking infra-red (FLIR) scanner using a cooled mercury cadmium telluride detector, or a cooled indium arsenide detector or even an uncooled micro-bolo metric array. The details of the scanner implementation are not important as long as:
a) the resolution is adequate,
b) the image acquisition speed is adequate (some thermal transients are of short duration)
c) the image can be acquired in the appropriate setting (real time acquisition for in plant production monitoring, remote portable and field worthy acquisition for in-situ applications).
d) the acquisition speed and mode is appropriate to the application (e.g. linear or flying spot scanning may be necessary for moving web processes, where as a real or snapshot acquisition may be necessary for quasi-stationary processes).
4. The scanned Far IR (3-10 micrometers wavelength of peak sensitivity) image is digitized and stored. The pixel resolution of the digitization and the storage system must be adequate to preserve the spatial resolution of the original IR data.
5. After a suitable time interval (this interval may vary from a fraction of a second in the case of a pipeline in use, to tens of minutes for large structures like the hulls of ships which have only been minimally heated), a second Far IR image of equivalent resolution is sampled and digitized. For the central part of the first and second images may be compared directly. For the marginal part thermodynamic modelling as described in the following steps may be used. If central and marginal parts are not designated then thermodynamic modelling is performed on the whole.
6. Standard thermodynamic modelling involving specific heats, conductivities, temperature differentials from ambient, and rough convection and other loss estimates is applied to the component data for the first sample, and the temperature distribution for a xe2x80x9cperfect homogeneousxe2x80x9d component at the instant of the second sampled is modelled and estimated. Alternately, this estimate may be derived from images of xe2x80x9cgoodxe2x80x9d articles taken at the second sampling time. The main purpose of calculating this estimate is to account for the significant non-uniformity of heat loss that arises directly from thermodynamics of the situation, so that comparison of the estimated temperature distribution with the actual will not high light any local anomalies.
7. The modelled radiant temperature profile estimate at the time of the second sampling is then compared with the actual profile data from the second sampling and the difference calculated, or high lighted.
8. Significant variation or anisotropies from within three dimensional structure then become evident. These may correspond to flaws or other non-uniformities.
9. The variations, and anisotropies evident in the image, can then be further enhanced using conventional image processing techniques, and:
a) presented in the form of a visual spot
b) quantified and used to make a pass/fail or grading decision.
It is believed the process of the invention is especially applicable to:
1. The inspection of pressed composite panel, such as OSB, or laminated products, in a production environment for anisotropies, resin spots and delaminations or other defects.
2. The in-situ inspection of structural panels on ships storage tanks, and other large structures; for external corrosion, paint or coating delamination, the buildup of layers or other defects.
3. The in-situ inspection of wall thickness variations in pipelines. In this case no marginal part is designated, or the marginal part involves only the ends of the pipes.